1. Field of the Invention
This invention relates to a control system for internal combustion engines for vehicles.
2. Description of the Related
Conventionally, a control system for internal combustion engines for vehicles is known, which reduces a shock generated during a so-called power-off downshift of an automatic transmission of an automotive vehicle, i.e., a downshift performed when an accelerator pedal of the vehicle is not depressed, by increasing the output torque of the engine to increase the rotational speed of the engine and hence shorten the inertia phase.
However, the conventional control system does not take into account variations in hydraulic pressure for operating the automatic transmission during the power-off downshift, and therefore provides the following inconvenience: FIG. 1A shows the relationship in timing between changes in the output torque of the engine, the hydraulic pressure, etc. during a power-off downshift, according to the conventional control system, in a case where the hydraulic pressure rises normally. On the other hand, FIG. 1B is a similar view to FIG. 1A, showing a case where the hydraulic pressure rises with delay.
According to FIG. 1A, hydraulic pressure Pon for operating a clutch of a lower-speed position (target speed position) rises without delay, and consequently the engaging force TP of the clutch becomes greater than the output torque TE of the engine at a time point a in FIG. 1A. As a result, the engine rotational speed NE increases to promote the downshift. On the other hand, according to FIG. 1B, the hydraulic pressure Pon rises with delay, and consequently the clutch engaging force TP is smaller than the engine output torque TE at a time point b in FIG. 1B. As a result, the engine rotational speed NE largely drops once at a time point c in FIG. 1B, so that the downshift temporarily retrogresses, resulting in a prolonged downshift duration. This prolonged downshift duration causes even a larger shock during the downshift, even if the engine output torque is increased by increasing the throttle valve opening TH as shown in the figure.
A possible reason for the rising delay in the hydraulic pressure for operating the lower-speed position (target speed position) clutch is that if the hydraulic pressure rises under a condition that transmission oil is completely drained from an oil passage of the automatic transmission, it takes some time period to again fill the oil passage with the transmission oil, which can delay the rise of the hydraulic pressure. Further, also the temperature or characteristics of the transmission oil can cause variations in the rise of the hydraulic pressure.